Since the 1800's fingerprint information has been collected from human fingers and hands by means of ink and paper. For the purposes of this document, the term fingerprint is used to mean the skin surface friction ridge detail of a single fingerprint, partial fingerprint or any portion of the skin surface friction ridge up to and including the entire hand or foot. In recent years various electronic fingerprint scanning systems have been developed utilizing optical, capacitance, direct pressure, thermal, and acoustic methods. Methods based upon acoustics, ultrasound, capacitance, and electric field measurement have proven to be the most accurate, as they are virtually immune to the effects of grease, dirt, paint, ink, and other image contaminants. Capacitance sensors may also offer additional advantage in that they may be able to achieve improved imaging in cases where poor acoustic impedance matching between the friction skin of the fingerprint and the scanner's platen are present, such as may be encountered when the skin on the finger is very dry.
The electric field method employs a transducer that capacitively couples the finger to an array of electric field measuring devices. The electric field may be a static field or one that employs a generating device that is coupled to the finger by contact with an electrode. Although the electric field is nearly uniform across the finger, there are variations in the electric field that give rise to differences in the measured electric field. For example, when a ridge of the friction skin of the finger is present, the measured electric field will be different than when a valley of the friction skin is present. Graphically displaying this information creates a contour map of the object (human finger or skin surface) that is in contact with the scanner surface. For example, the depth of any gap structure, such as the ridges and valleys of the fingerprint, may be displayed as a gray-scale bitmap image. Measuring the electric field via the capacitance coupling to the platen surface makes use of the fact that the electric field is a function of the distance between capacitance plates, i.e., the TFT input pad and the skin of the finger. Ridges of the fingerprint are closer to the input pad and valleys are places where the skin is farther away from the TFT input electrode pad, and thus differing electric field measurements that can be used to identify the location of the ridges and valleys of the fingerprint.